Reference flow liquid chromatography system with stable baseline

ABSTRACT

There is disclosed herein an apparatus and method for performing liquid chromatography analysis with a stable baseline. The apparatus utilizes positive displacement means on each of two flow paths and a control system to regulate the flow such that the flow in both flow paths is identical. Only one flow path has sample injected therein. Each flow path is passed through a separate liquid chromatography column and a common detector at the output end of each column. Since flow path contains solvent only and the flow path contains both solvent and sample, any changes in the characteristics of the output streams of the liquid chromatography columns caused by changes in the solvent composition itself have a common mode. Therefore, only the difference signal is output from the detector which indicates changes in the characteristics of the output stream of the sample flow path caused solely by the presence of the sample components. By outputting the signal from the detector, a stable baseline may be achieved which takes into account only changes caused by the sample components and not changes caused by alteration of the solvent composition. This allows refractive index detectors to be used with gradient solvent analysis for the first time.

This application is a division of application Ser. No. 456,021, filedDec. 22, 1989, which was a file wrapper continuation of Ser. No.155,592, filed Feb. 12, 1988, now abandoned.

BACKGROUND OF THE INVENTION

The invention pertains to the field of liquid chromatography, and, moreparticularly, to the field of pumping systems to control the flow ofsolvent and sample through liquid chromatography columns.

Liquid chromatography is a process whereby the known components of asample may be analyzed to determine the quantity of each component in asample of unknown composition. To make such an analysis, the sample isdissolved in a solvent stream which is then passed through the liquidchromatography column. Liquid chromatography columns contain particlesoften modified with chemical reagents which act upon the solvent andsample system to retain the various sample components for differentamounts of time. In other words, as the sample is pumped through thecolumn, the output stream is comprised of the basic solvent whichcarries the sample plus the individual components of the sample whichemerge from the output of the column at different times.

Liquid chromatography analysis systems detect the presence of thevarious components of the sample in the output stream using varioustypes of detectors. The purpose of these detectors is to recordcharacteristics of the output streams and to generate signals in theform of output waves (spikes) which represent the presence and quantityof each particular sample component. Each of these spikes appears in theoutput stream at a different time. The time of occurrence of each spikeindicates which component of the sample is then emerging. These"retention" times are based upon the known characteristics of eachsample component and the known characteristics of the liquidchromatography column in acting upon that component. However, theseretention times are also based upon assumptions as to the magnitude ofthe flow rate of solvent through the column and the relative stabilityof this flow rate.

Normally, light absorbance detectors are used at the output of theliquid chromatography column to detect changes in light absorbance assample components emerge. These absorbance detectors depend upon thelight absorbance of the sample components in the output stream andgenerate signals indicative of this absorbance which can be used toquantify the size of the peak.

However, in certain applications, the sample components do not absorblight. In these applications, detectors such as refractive index,fluorescence, conductivity, electrochemical and other types of detectorsare used to quantify the peaks. Refractive index detectors measure thevarying amounts of refraction of a light beam as it passes through theoutput stream. The amount of refraction or bending of the light beam atany particular time as it passes through the output stream depends uponthe refractive index and the magnitude of a particular sample componentin the output stream at that time. These detectors output signals whichquantify the amount of refraction.

An unfortunate side effect of the use of refractive index detectors isthat they are very sensitive to changes in the composition of thesolvent carrier. In many applications, it is useful to have solventcompositions with multiple components, and frequently "gradient"solvents are desirable. A gradient solvent is a solvent comprised of twoor more solvents where the solvent composition is varied over time. Avery popular way of making up gradient solvents is to have each solventdelivered by a separate pump and to run each pump at an appropriatespeed to make up the currently desired solvent composition.

Gradient solvent compositions are not used in the prior art whererefractive index detectors must be used. Additionally, the use of pumpblended solvent mixtures cannot occur with refractive index detectors.The only current method of forming the solvent composition for use withrefractive index detectors is to form the solvent composition in aseparate container and mixing it thoroughly. This separate process offorming the solvent composition and mixing it thoroughly causes therelative makeup of the solvent composition to be known and non-varying.

However, this is an inconvenient process for use in commercial liquidchromatography systems and is incompatible with creation of acontinuously changing gradient. In commercial liquid chromatographysystems, solvent composition makeup is normally done in either of twoways. One way involves using a plurality of valves which gate thevarious solvent components into the pump which drives the solventcomposition through the liquid chromatography column. Another way isthrough the use of multiple pumps as described above.

However, the reproducibility of the solvent composition which can beformed using either the multiple pump or valve method is not sufficientfor use with refractive index detectors. This is because the pumps orvalves cannot control the exact composition of the solvent mixture aswell as by hand mixing. Most pumps have a repeatability factor of 0.1%which is not high enough for use with refractive index detectors.

Basically, refractive index detectors are so very sensitive to thesolvent composition that even the slightest error in the relativemagnitude of the quantities of solvent components in the solvent mixturewill lead to a phenomenon called "baseline drift". Baseline drift refersto changes in the output signal of the detector which are caused bychanges in the solvent composition and not changes in the samplecomponent content of the output stream. Basically, baseline drift isnoise which degrades the accuracy of the results which can be obtainedby a liquid chromatography system. In an ideal system, the output of thedetector at the output of the liquid chromatography column would be asteady state stable value when no sample was injected into the inputstream entering the column. However, when solvent makeup valves orindividual pump motors for each solvent component are used to make upthe solvent composition, the errors in the composition which resultcause the baseline to drift erratically because of the extremesensitivity of the refractive index detectors to the slight changes inthe refractive index of the solvent composition itself. The same problemcan occur with use of other detectors, but is aggravated when usingrefractive index detectors.

Another problem with the use of pumps for solvent makeup is that thesolvent composition is often not thoroughly mixed. This results inerratic baseline drift also.

Further, with absorbance types of detectors, the absorbance of thesolvent itself changes at different wavelengths. This too can result inbaseline drift error.

One solution to baseline drift that has been tried in the prior art isto use an insensitive range on the recorder used to record the resultsof a run. Unfortunately, this limits the sensitivity and resolution ofthe system. Thus, the use of gradient solvents with refractive indexdetectors has not been possible in the past and the use of pumps forsolvent makeup has not been possible when using refractive indexdetectors even when gradients were not being used. Further, baselinedrift is also a problem with other types of detectors such as absorbancedetectors where the wavelength of the light is sensitive to theabsorbance characteristics of the solvent components.

Others have attempted to solve the problem of baseline drift in theprior art but have failed. At least two groups of workers in the arthave tried dual flow chromatography systems whereby the stream ofsolvent is split by a T connection in front of the column into twopaths. One of these two paths was routed through the column while theother flow path was coupled directly to the common detector referenceflow path input. This approach is described by Hunkapillar and Hood inScience magazine, Vol. 207, p. 24 (1980) and by Stevenson and Burtis inClinical Chemistry, Vol. 17, page 774 (1971). The problem with thisapproach is that no positive displacement means is provided for eachpath. Thus, if differences in the resistance to flow exist between thetwo flow paths, unequal flow rates exist in each path, and the referencesignal will become "out of synchronization" with the sample signal forwhich the reference signal is supposed to act as a reference. This causeerrors.

Accordingly, there has arisen a need for a liquid chromatography systemwhich is free of baseline drift in all applications. Such a systemshould be able to mix solvent gradients using pumps or solvent makeupvalves to make either gradient or constant solvent compositions and beusable with any type of detector including refractive index detectorswithout errors caused by baseline drift.

SUMMARY OF THE INVENTION

According to the teachings of the invention, there is disclosed herein amethod and apparatus for performing either gradient or non-gradientsolvent analysis of unknown samples using liquid chromatography systemswith any type of detector including refractive index detectors. Thebasic principle of the invention is the use of two positivedisplacement, equal flow branches through two independent liquidchromatography columns each of which flows into a common detector whichdetects the difference between the two output streams characteristicsand outputs a signal indicative of the difference. One flow is calledthe reference flow path and has no sample injected therein and the otherflow is called the sample flow path and has sample injected therein.Both paths should be the same length. Because the flow rate in each pathis forced to be the same, any error caused by baseline drift in theoutput of the sample flow carrying the unknown sample can be eliminatedby comparison with the reference flow path.

The method according to the teachings of the invention is to use anymeans possible to force the flow rate in the two paths through twoindependent liquid chromatography columns to be the same. Thus, eventhough one liquid chromatography column may have more back pressure thanthe other, this resistance to the flow is overcome by the positivedisplacement apparatus in each path thereby forcing the flow rates to bethe same.

The apparatus according to the teachings of the invention may haveseveral different embodiments. In the preferred embodiment, a singlepump having two separate flow paths is used. This pump may have one, twoor three heads. In the two head design, two pistons in the first pumphead are input pistons which draw their solvent composition through a Tconnection from a single solvent composition makeup system comprised ofa plurality of makeup valves in a mixing block and a mixture controlinput for receiving mixture control information. Two check valves forthe input piston pair insure that solvent flow is unidirectional intothe input pistons and out from the output pistons. The outputs of thesetwo input pistons are individually directed through output check valvesto a pair of output pistons in the second pump head. The output of eachof these output pistons is connected to the individual liquidchromatography columns in the two independent flow paths. That is, oneof the output pistons drives the solvent composition through one of theliquid chromatography columns, while the other piston drives the solventcomposition through the other liquid chromatography column. There is asample injector which injects sample in the solvent stream entering oneof the liquid chromatography columns.

The output of each of the liquid chromatography columns is connected toa common detector to detect the peaks occurring in the output stream.The detector uses a simple differential amplifier circuit to subtractthe output of the reference flow path from the output of the sample flowpath thereby achieving accurate results without baseline drift. Areduction in baseline drift by a factor of 90 has been achieved usingthe teachings of the invention.

Alternative embodiments for the structure include the use of twoseparate two-piston pumps which are mechanically driven on the sameshaft by the same motor. Each of the pumps drives a separate flow path,but each pump draws its input solvent from a common source.

Another alternative embodiment is the use of two separate pumps whichare electonically controlled to have the same flow rate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing, in schematic form, a generic versionof the apparatus of the invention.

FIG. 2 is a sample plot of a detector baseline response of the output ofeither an absorbance detector or a refractive index detector over timewhen a perfectly formed gradient is slowly altered during the course ofa run showing the effect of baseline drift caused by changing absorbanceand changing refractive index as the solvent composition changes.

FIG. 3 is a sample plot of a refractive index detector baseline responsewith no sample present where either multiple separate pumps or a singlepump with valves is used to make up the solvent composition.

FIG. 4 is a block diagram of the preferred embodiment of the apparatusof the invention wherein a single pump is used with two pump heads, eachpump head having two pistons, with both pump heads being driven by asingle motor thereby causing two equal but independent flow rates to beachieved.

FIG. 5 is a block diagram of an alternative embodiment of the apparatusof the invention wherein two pumps are used, each pump having two pumpheads, each pump head having a single piston and wherein each pump ismechanically linked to the other pump such that a single motor drivesall pistons in both pumps at the same speed.

FIG. 6 is a block diagram of another alternative embodiment of theinvention wherein two pumps are used with each pump having a pair ofpump heads, each pump head having a single piston and wherein each pumpis driven by the same flow control system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, there is shown a block diagram of a generic systemaccording to the teachings of the invention. A solvent compositionmakeup block 10 is coupled to a plurality of different solvent sources.This composition makeup block can be any conventional apparatusincluding a single pump drawing its input from a plurality of solventcomposition makeup valves such as the SP8800 Gradient Pump availablecommercially from the assignee of the present invention. Each of thevalves is coupled to a different solvent source. In alternativeembodiments, the composition makeup block 10 can be a plurality ofindividual pumps, each drawing its input from a different solvent sourceand each being run at a speed which is a function of the desiredpercentage of that solvent component in the final composition. A desiredcomposition input signal on line 12 defines the desired percentage ofeach individual solvent element in the final composition in outputstream 14. The desired composition signal may be a packet of digitaldata comprised of one or more bytes defining the desired percentage of aparticular solvent composition element. In other embodiments, thesolvent composition makeup information may be a plurality of analogsignals, each on a separate line and each controlling the degree ofopening of a plurality of corresponding linear, solenoid operatedvalves.

In some embodiments, the solvent sources on lines 16, 18, etc. may bepressurized either by the application of above atmospheric pressure toeach solvent reservoir or by the use of a pump connected to each of thesolvent input lines 16, 18, etc.

A pair of back-pressure-independent, flow control systems 20 and 22 drawtheir inputs from the solvent composition makeup system 10. The outputstream 14 from block 10 is split by a T connector 24 into two streams 26and 28. These two streams are coupled to the inputs of the flow controlsystems 20 and 22.

Each of the flow control systems 20 and 22 is a positive displacementsystem which controls the output flow on lines 30 and 32 independentlyof back pressure exerted by the liquid chromatography columns 34 and 36or other restrictions in the flow path. Column 36 need not be achromatography column. It is only necessary that a structuresubstituting for column 36 substantially reproduce the characteristicsof column 34 such as internal volume and mixing characteristics. Each ofthe flow control systems 20 and 22 receives control signals via acontrol bus 28 which defines the desired flow rate on output lines 30and 32.

The structure of the flow control systems 20 and 22 can be any structurewhich insures that the flow rates on lines 30 and 32 are identical. Thespecific examples of the structures of these flow control systems willbe described later herein. Any structure which can accept the desiredflow rate signal on line 38 and can cause identical flow rates to occuron output lines 30 and 32 regardless of differences of back pressure andother flow restrictions on these lines will suffice for purposes ofpracticing the invention.

In FIG. 1 the sample flow path is at the top of the figure and thereference path is at the bottom of the figure. These two flow paths musthave the same path length. The sample flow path includes a sampleinjector 40 of conventional design. The purpose of this unit is toinject the unknown sample composition entering the injector on line 42into the incoming solvent composition stream on line 30 such that thesample is entrained in the input stream on line 44 to the liquidchromatography column 34. Normally, no sample injector 40 is found inthe reference flow path since the purpose of the reference flow path isstrictly to provide a reference signal for base line stabilization.Special applications could require a sample injector in the referenceflow path. This reference signal is generated by detector 50 and isbased solely on the characteristics of the solvent composition itself inline 32 after it emerges from the liquid chromatography column 36 orother device substituting for liquid chromatography column 36 on line48. It is to be understood that references to first and second liquidchromatography columns made elsewhere herein also refer to a firstliquid chromatography column in the path containing the sample and asecond flow path simulating at least the path length of the liquidchromatography column as the reference flow path.

Detector 50 in both the sample and reference flow paths examines thecharacteristics of the emerging liquid streams on lines 48 and 52 fromthe liquid chromatography columns 34 and 36. Detector 50 may be eitherlight absorbance type of detector or the refractive index type ofdetector. Any other form of detector used in liquid chromatographyanalysis may also be used. Suitable detectors include but are notlimited to conductivity, electrochemical, infrared, photodiode arraymultiwavelength, fluorescence, and the like. In the preferredembodiment, the detector 50 is either model number 8450 or 8490available commercially from the assignee of the present invention.

The purpose of detector 50 is to detect the presence of individualcomponents in the sample composition from line 42 as these individualcomponents emerge at different times from the liquid chromatographycolumn 34 and compare the responses to those of the reference flow pathin line 48 and column 36. This detector detects these components by thechanging light absorbance in the stream 52 or by the changing refractiveindex of this stream. Detector 50 outputs a signal on line 54 which isthe difference between the optical characteristics of the stream in line52 and the stream in line 48.

Since these optical characteristics may be varying because of variationsin the input solvent composition on line 30, the signal on line 54 mayhave noise in it which can cause errors. The source of this noise isillustrated in FIGS. 2 and 3.

FIG. 2 illustrates the baseline drift in detector response over timewhen a perfectly formed gradient solvent composition has its compositionchanged during the course of a run with no sample present. FIG. 2represents the detector response to changes in both the refractivecharacteristics and the absorbance of the solvent composition standingalone with no injected sample as the solvent composition is slowlychanged under the control of the user. A gradient means that therelative proportions of solvents 1, 2, 3, etc. are changing. Becauseboth the refractive index and the light absorbance properties of thesolvent composition changes when the relative proportion of itsindividual components change, it is seen from FIG. 2 that the baselineresponse of the detector to such a changing or gradient solventcomposition is not a flat response but changes over time. Function 54 inFIG. 2 represents the baseline drift error caused by gradient solventcomposition. Function 54 shows only the above stated source of errorabsent any other source of error such as non-repeatability in thesolvent makeup process which is represented in FIG. 3 and is anindependent source of error. The ideal flat baseline response is shownby the phantom line at 56.

With a changing baseline like that shown at 54, the characteristics ofthe baseline itself are "modulated", i.e., superimposed upon, thechanges in the refractive index or absorbance caused by the variouscomponents of the sample as they emerge from the liquid chromatographycolumn at different times. Since the only changes of interest are thosechanges caused by the presence of sample components in the outputstream, having a changing baseline causes errors in correctlyinterpreting the optical characteristics of the output stream from theliquid chromatography column. If the effects of the changing baselinecannot be removed from the output of the detector so that only changescaused by the sample components are present, then gradient solventcompositions cannot be used. This is the case in the prior art withrefractive index detectors. These refractive index detectors areextremely sensitive to changes in the solvent composition itself. Unlessthe solvent composition has a very stable makeup which does not change,a refractive index detector will inject noise into the output signalcaused by the changes in the solvent composition.

In many prior art applications, the solvent composition is set andchanged either by controlling the speed of individual motors which pumpthe various individual components or by controlling the timing of valveson a single pump. The solvents go into a mixer where they are mixed intothe final solvent composition. Sometimes the mixer is omitted which canbe an independent source of error where incomplete mixing results. Evenwhen perfect mixing occurs, pumps or valves have errors which can causeanother source of error. When using individual pumps for solvent makeup,the speeds of the motors and accuracy of pumping of each individualsolvent component cannot be controlled closely enough with sufficientlyrepeatable accuracy to prevent errors. When using a single pump withvalves, the lack of repeatability of the valves introduces errors. Whenusing either individual pumps or a single pump with valves for solventmakeup, as is frequently done in the prior art, a baseline such as thatshown in FIG. 3 often results. This source of baseline error is theslightly changing composition of the solvent even where gradients arenot being used because of pumping or valve inaccuracies. These slightchanges are reflected in the output signals of the detectors as baselinedrift errors. This is especially true in the case where refractive indexdetectors are used since such detectors are extremely sensitive to themost minute of changes in the solvent composition. Thus, use ofindividual motors to pump each solvent component or a single pump withvalves or a single pump with valves is unworkable to make up the solventcomposition even where no gradient is to be used. The baseline drifterror of the type shown in FIG. 3 will be superimposed upon the baselinedrift of the type illustrated in FIG. 2.

Referring to FIG. 4, there is shown a schematic diagram of the preferredembodiment of a dual flow liquid chromatography analysis systemaccording to the teachings of the invention. Solvent composition makeupis provided by a mixing block 60 which is coupled by three input lines62, 64, and 66 to three different solvent sources. Of course, more thanthree or less than three solvent sources may be used. The mixing blockcomprises a plurality of linear solenoid operated valves or otherdevices to control the relative amounts of each different solvent thatare fed into output line 68. The relative proportion of each solvent iscontrolled by signals on the bus 70. The structure of the mixing block60 is conventional, and any known solvent composition makeup structurewill suffice for purposed of practicing the invention.

The solvent composition on line 68 is not always well mixed.Accordingly, an optional mixer 70 receives the solvent composition andmixes it thoroughly for output on the line 72.

To provide positive displacement and flow control on each of theseparate flow paths, a pump 74 is provided. This pump is comprised oftwo separate pump heads in the preferred embodiment. An input pump headat 76 contains two input pistons 78 and 80. Each of these input pistonsdraws a separate stream of solvent through a T connection 82 coupled tothe output of the mixer 70. These two input streams on lines 84 and 86are drawn by the input pistons through two one-way input check valves 88and 90. These check valves insure that when the input pistons 78 and 80undergo their compression strokes, flow of solvent emerges from the pumphead 76 on the output lines 92 and 94 and not on the input lines 84 and86.

The pump 74 also has an output head 96 which contains two output pistons98 and 100 in the preferred embodiment. These two output pistons drawtheir input solvent stream from the lines 92 and 94 through output checkvalves 102 and 104. These two output check valves insure that flow ofsolvent is in one direction only from lines 92 and 94 to output lines106 and 108, respectively. Both pistons in each individual pump head arethe same size and are driven at the same speed. The speeds of thepistons in the input pump head may be different from those in the outputpump head. In addition, the input pistons 78 and 80, if driven by a cam,are driven either by the same cam or by two cams having identicalprofiles and turning at the same rate and indexed identically. The samestatements are true for the output pistons 98 and 100.

A transducer 110 in output line 108 senses either the flow rate or thepressure in line 108 and provides a signal indicative thereof on line112. Line 112 is a control input to a flow rate control system 114 whichwill be described in more detail below. The purpose of the flow ratecontrol system 114 is to compare the actual flow rate to the desiredflow rate and to control the driving system for the pistons so that thedesired flow rate is achieved in both flow paths. Identity of flow ratesautomatically results because the pair of pistons 78 and 80 are the samesize, run at the same speed and ride the cam profiles as defined above.The same comments hold for pistons 98 and 100. Any structure for theflow rate control system 114 that will achieve this result will sufficefor purposes of practicing the invention. Suitable structures includebut are not limited to analog flow rate control, digital flow ratecontrol, combinations thereof, and the like.

Lines 108 and 106 symbolize the two separate flow paths of the liquidchromatography system. One of these flow paths, the one symbolized byline 108 is the sample flow path while the other, symbolized by line106, is the reference flow path. Both flow paths should be of the samelength for optimum performance. The sample flow path has the sample ofunknown composition injected therein. This is accomplished by aconventionally designed sample injector 116 of conventional structure.The sample of unknown composition enters on line 118 and is entrained inthe solvent composition entering via line 108. The output from thesample injector on line 120 is input to a first liquid chromatographycolumn 122. The liquid chromatography column is of conventional design.

The reference flow solvent composition on line 106, without the sampleinjected therein, is passed through a second liquid chromatographycolumn 124.

The flow rates in the lines 106 and 120 are caused by the flow ratecontrol system to be the same regardless of whether the liquidchromatography columns 122 and 124 individually imposed different backpressures resisting the flow of solvent therethrough. Accordingly, ifthe solvent composition in line 68 is changing for any reason, the verysame changing solvent composition will emerge from the two liquidchromatography columns 122 and 124 on lines 126 and 128 simultaneously.

Detector 130 is used to detect the characteristics of the liquids inlines 126 and 128. Detector 130 may be any type of detector including arefractive index detector. Detector 130 detects not only the changes inthe characteristics in the fluid in line 126 caused by the sample ofunknown composition but also the changes caused by changes in thesolvent composition itself. Detector 130 also detects the changes in thecharacteristics of the fluid in line 128 caused by changes in thesolvent composition.

The detector 130 generates an output signal on line 134 which definesthe changes in the characteristics of the fluids in lines 126 and 128,respectively. The output 134 is between fluid stream 128 and fluidstream 126. When there is no sample injected by the sample injector 116,the signal on line 134 will be a stable baseline like that shown at 142in the sample plot of a typical detector output shown in the lower righthand corner of the figure. The dash-dot line 144 represents the baseline which would result with no sample injected were it not for the useof two separate flow paths including a reference flow path. Thus, it canbe seen that the presence of the second, i.e., reference flow path,causes the base line 142 to be made artificially flat. This results inchanges on the detector output axis 146 resulting only from peaks causedby the components of sample as they emerge on the line 126.

The flow rate control unit 114 can take several different alternativeembodiments. In the single pump embodiment shown in FIG. 4, the inputhead 76 and the output head 96 are both part of the same pump and aredriven by the same motor and shaft. Therefore, the flow rate controlunit 114 represents a conventional flow control circuit for such a pump.Such flow control systems are known and are commercially available fromthe assignee of the present invention in the form of the flow ratecontrol system for the MINI-PUMP™ liquid chromatography pumping system.This control system is described in copending U.S. patent applicationAPPARATUS FOR CONTROLLING A PUMP TO ACCOUNT FOR COMPRESSIBILITY OFLIQUIDS IN OBTAINING STEADY FLOW, by Honganen et al., Ser. No. 913,356,filed 9/30/86. In fact, the pump 74 may be the MINI-PUMP liquidchromatography pump commercially available from the assignee if it ismodified by adding an additional piston to each pump head. The pumpheads may need to be enlarged for this modification since very highpressures are used in liquid chromatography systems which may lead toexcessive generation of heat beyond the capacity of the MINI-PUMP pumpheads to provide adequate cooling. Those skilled in the art of pumpdesign will appreciate the modifications that need to be made to add anadditional piston to each pump head. It is only necessary for purposesof practicing the invention that individual pumping action in each flowpath be provided such that the flow rate in each flow path can bematched regardless of differences in the back pressure in each flowpath. In the embodiment shown in FIG. 4, this automatically happenssince both of the input pistons have the same size and are driven on thesame shaft by the same cam configuration. The same is true for theoutput pistons 98 and 100. The output pistons are of the same size andare driven by the same motor shaft as the input piston 78 and 80.Therefore, the flow rate control system 114 merely needs to control thesingle pump motor for the desired flow rate in the line 108, and thisflow rate will automatically be obtained in line 106 by virtue of thefact that the pistons driving fluid through line 106 are the same sizeas the pistons driving fluid through the line 108.

FIG. 5 illustrates an alternative two pump embodiment with a mechanicallinkage between the pumps so that a single motor drives both pumps atthe same speed. In this embodiment a first pump 150 may be a MINI-PUMPliquid chromatography pump or it may be any other liquid chromatographypump which is capable of achieving the desired flow rate and capable ofbeing mechanically linked to the drive shaft of another liquidchromatography pump 152. The second pump 152 may be of the same designas pump 150 or a different design. It is only necessary in theembodiment shown in FIG. 5 that each pump be capable of delivering thedesired flow rate that the two pumps be capable of being mechanicallydriven by the same motor such that both pumps deliver identical flowrates in the two flow paths independent of differences in back pressurewhen driven at the same speed. A solvent makeup block 154 has threeinputs 156, 158, and 160 which are coupled to the sources of threedifferent solvents. Of course, more than three solvents or less thanthree solvents may also be used by suitable alteration of the solventmakeup block valving mechanism. As in the case of the embodiment shownin FIG. 4, solvents one, two and three may be pumped or may bepressurized. However, it will be appreciated that solvent one, two andthree may be at atmospheric pressure and not pumped or pressurized.

The output from the solvent composition makeup block on line 162 is fedto a conventional mixer 164. The output of this mixer on line 166 iscoupled to a T connector 168. This T connection splits the stream ofsolvent on line 166 into two flow segments on lines 170 and 172. Line170 is coupled to the input of pump 150. Line 172 is coupled to theinput of pump 152. The flow rate in both lines 170 and 172 will be thesame because each of the pumps is pumping at the same flow rate.

A single motor 174 drives pump 152. This motor drives two cams (notshown) in pump 152. One cam drives an input piston 176 and the other camdrives an output piston 178. The cams may have different profiles, butboth are driven at the same rotational speed.

A mechanical link 180 is coupled to and rotates with the cams whichdrive the input pistons 176 and 178 of pump 152. Mechanical link 180 iscoupled to two cams (not shown) in pump 150. One cam drives an inputpiston 182 and the other cam drives an output piston 184. The cam thatdrives the input piston in pump 150 has the same profile and is indexedidentically as the cam that drives the input piston in the pump 152.

The two input pistons 176 and 182 draw solvent from lines 170 and 172 inat the same rate through input check valves 186 and 188. Both of theseinput pistons pump solvent out through output check valves 190 and 192to output pistons 184 and 178, respectively. The check valves 186, 190,188, and 192 insure that solvent flow is unidirectional in each of thepumps in the stated direction. The output pistons 184 and 178 receivesolvent from the input pistons via lines 194 and 196, respectively.These output pistons then pump solvent out through the sample flow path198 and the reference flow path 200.

The sample flow path has sample of unknown composition on line 202injected therein by a sample inject block 204. A transducer 206 sensesflow conditions in the sample flow path and outputs a signal on line 208to a flow control system 210. The flow control system receives a flowcontrol signal on line 212 regarding the desired flow rate. The flowcontrol system then calculates the actual flow rate as indicated by thesignal on line 208, and compares the actual flow rate to the desiredflow rate to generate an error signal which is used to control the motor174 via line 214. Although flow conditions are only sensed in the sampleflow path 198, since pumps 150 and 152 are driven by the same shaft, thesame flow rate will result in both the sample flow path and thereference flow path regardless of different flow restrictions in eachpath.

Referring to FIG. 6, there is shown an alternative embodiment of thesystem of the invention. In FIG. 6 the various elements have the samepurpose and function as the elements with the same numbers in theembodiment shown in FIG. 5. The only difference between the embodimentsof FIG. 6 and FIG. 5 is that in FIG. 6 there is no mechanical linkagebetween the first and second pumps 150 and 152, respectively. Equal flowrates in the sample flow path 198 and the reference flow path 200 areachieved using a single flow control system 210 which compares thedesired flow rate signal on bus 212 to the actual flow rate in thesample flow path as transmitted on line 208 by a transducer 206. Theflow control system can be the same known structure defined in the abovecited U.S. patent application which is hereby incorporated by reference.The flow control system then compares the actual flow rate to thedesired flow rate and generates an error signal on bus 214 which is usedto control a motor 174 driving pump 152 and a motor 175 driving pump175. In the preferred version of this embodiment, pumps one and two areMINI-PUMPs and are commercially available from the assignee of thepresent invention. The motors 174 and 175 are stepper motors. The flowcontrol system 210 and the flow control signals on the bus 214 as wellas the feedback signal from the sample flow path transducer 206 are asdescribed in above cited U.S. Patent Application.

Because both motors are driven with the same control signals, each motordrives its pump at the same speed. Because the pumping pistons and eachof the cylinders are the same size and are being driven at the samespeed with identical cam profiles for corresponding pistons in eachpump, equal flow rates are achieved in the sample flow path and thereference flow path.

Although the invention has been described in terms of the preferred andalternative embodiments disclosed herein, those skilled in the art willappreciate numerous other modifications which may be made withoutdeparting from the teachings of the invention. All such modificationsare intended to be included within the scope of the claims appendedhereto.

What is claimed is:
 1. A method of pumping liquid through a liquidchromatography system comprising:making a solvent composition; dividingthe solvent composition into first and second streams; injecting asample having at least some unknown characteristics into said firststream; pumping said first and second streams through independent firstand second flow paths having identical flow path lengths with individualpumping piston and cylinder combinations in each of said first andsecond flow paths at points in each of said first and second flow pathswhich are not common to said first and second flow paths, each said pumppiston and cylinder combination being operated so as to cause identicalflow rates to exist in each of said first and second flow pathsregardless of changes in resistance to flow which may occur in either ofsaid first or second flow paths, said first flow path containing aliquid chromatography column and said second flow path containing adevice initially having the same flow path length and back pressure assaid liquid chromatography column; detecting the differences in thecharacteristics of the liquid streams emerging from said first andsecond flow paths; and outputting a signal indicative of the differencesin characteristics between said first and second streams.
 2. The methodof claim 1 wherein the step of making up the solvent compositioncomprises driving at different speeds each of several motors each ofwhich pumps a different solvent into the composite solvent streamentering the means for pumping, where the speed of each motor is setrelative to the speeds of all the other motors such that the desiredsolvent composition is achieved.
 3. The method of claim 2 wherein thestep of detecting the characteristics of the first and second streamscomprises the steps of measuring the refractive index of each streamfrom time to time and generating an output signal indicative thereof. 4.The method of claim 3 wherein the step of determining the differences inthe characteristics of the output streams comprises the step ofdetermining the difference between the magnitudes of the signals fromthe detector used to measure the refractive index of each of said firstand second streams.
 5. A method of pumping liquid through a liquidchromatography system having first and second flow paths containingfirst and second liquid chromatography columns, respectively, said firstand second liquid chromatography having identical flow path lengthscomprising:making a solvent composition; dividing the solventcomposition into first and second streams; injecting a sample having atleast some unknown characteristics into said first stream; pumping saidfirst and second streams through independent first and second liquidchromatography columns, respectively, using first and second pumpscoupled at points in said first and second flow paths which are notcommon to said first and second flow paths, to said first and secondliquid chromatography columns, respectively, said first and second pumpsbeing operated so as to maintain identical flow rates through said firstand second liquid chromatography columns regardless of changes in theback pressure resisting flow which occur in either of said first orsecond liquid chromatography columns; detecting the opticalcharacteristics of the liquid streams emerging from said first andsecond liquid chromatography columns using refractive index detectorsand generating signals indicative of the refractive index of each ofsaid streams emerging from said first and second liquid chromatographycolumns; and determining the differences between said signals from saidrefractive index detectors thereby indicating the differences in opticalcharacteristics between the two streams as said streams emerge from saidliquid chromatography columns, and outputting a signal indicative ofsaid differences.